1. Field of the Invention.
The invention relates to adjustable radiant heat barriers in heaters.
2. Description of the Related Art.
When a body is placed in an enclosure whose walls are at a temperature above that of the body, the temperature of the body will increase even if the enclosure is evacuated. The process by which heat is transferred from an enclosure by virtue the temperature of the enclosure, without the aid of any intervening medium, is called thermal radiation. The emission of thermal radiation is governed by the temperature of the emitting enclosure. The range of wavelengths of thermal radiation is approximately 0.1 microns to 100 microns.
Direct-fired heaters with fluid tubes on the outside walls of the heater are commonly used in various industrial applications. The fluid in the fluid tubes is often water, hydrocarbon fluid, or some other type of heat transfer fluid. Direct-fired heaters have an interior combustion chamber in which fuel and oxygen combine in the presence of an ignition source to form a flame. The flame, the flue gases and other products of combustion produce electromagnetic radiation which heats the fluid in the fluid tubes. Although heat conduction and convection also occur in the combustion chamber, the primary mode of heat transfer from the flame to the fluid tubes is usually radiative heat transfer.
When radiant heat from the flame and the combustion gases reaches the outer surface of the fluid tubes, this outer surface absorbs, transmits and reflects the radiant heat. For opaque surfaces such as fluid tubes, the amount of radiant heat directly transmitted by the fluid tube outer surface is not significant. The heat absorbed by this outer surface is then re-emitted or transported away from the outer surface to the fluid inside the tubes by a combination of conduction and convection. The heat conducted away from the outer surface of the fluid tubes passes through the solid material of the fluid tubes to the inner surfaces of the tubes. At this inner surface, fluid flowing through the fluid tubes absorbs the heat and transports it downstream, eventually exiting the portion of the fluid tubes within the heater.
In most applications, the fluid tubes are connected to a piping system where it is desirable to maintain the fluid temperature at a design temperature at the outlet from the fluid tube bank to achieve the process or heating objectives.
The most common method for maintaining the fluid temperature in the piping system at the design temperature is to change the flow rate of fuel gas and combustion air to the heater. With a lower flow rate of fuel gas, the heater enclosure will be lower and less heat is transferred from the flame to the fluid tubes.
In many processes, the fluid is sensitive to the rate at which heat is transferred through the fluid film located at the inside wall of the fluid tube. If the fluid film is overheated, thermal degradation or damage to the physical properties of the fluid may occur. Overheating of the fluid film may also cause fouling of the tube walls, which prevents the fluid from effectively convecting heat from the tube wall. As a result, the tube wall temperature may exceed design temperatures for the fluid tube material and eventually result in mechanical failure of the material.
It is, therefore, desirable to have a system and a method for maintaining the desired total heat input and fluid design outlet temperature, without exceeding the design fluid film temperature or a mean temperature of the fluid tube. One way of controlling the fluid design temperature is to control the rate of heat transfer from the heater flame to the outer surfaces of the fluid tubes.